First, are there any mail servers that you are already familiar with?
Second, what are the requirements for the project?
What operating system is required or can you best support?
How many users are there? How many domains are there?
How will they be accessing their email? Internal only, pop3, webmail?
Do they need smtp-auth access so they can send email through the servers from home?
It sounds like this is not a big budget operation. You'll want to plan out the details and know exactly what's required before getting started. To meet the redundancy requirements you can have 2-3 redundant servers that all handle incoming email and filtering (virus scanner, spam filtering, etc) and then a better class server with hardware redundancy within itself (mirrored hard drives, dual power supplies, etc) that you can use as a backend NFS and storage server. The numbers of servers here might be off. Is there any existing information about users, number of incoming emails per hour during business hours (usually the peak hours), etc?
Don't forget including a backup solution in the project plan!